1. Field of the Invention
The present invention relates to a catalyst deterioration degree acquiring apparatus that is disposed in an exhaust passage of an internal combustion engine and has an oxygen storing function.
2. Description of the Related Art
In general, a catalyst (three-way catalyst) disposed in an exhaust passage of an internal combustion engine and has an oxygen storing function is deteriorated as a result of poisoning by lead, sulfur, or the like contained in a fuel, or heat applied to the catalyst, whereby the maximum oxygen storing quantity, which is the maximum value of the oxygen storable in the catalyst, decreases, as the deterioration of the catalyst progresses. In other words, the maximum oxygen storing quantity can be an index indicating the degree of the deterioration of the catalyst. Therefore, if the maximum oxygen storing function of the catalyst can be estimated (detected), the degree of the deterioration of the catalyst can be acquired on the basis of the estimated maximum oxygen storing function.
On the basis of this knowledge, the apparatus disclosed in Japanese Patent Application Laid-Open No. 2003-97334 estimates the maximum oxygen storing quantity in the manner described below. Specifically, the air-fuel ratio of the gas flowing into the catalyst is controlled to be a predetermined rich air-fuel ratio that is on the richer side with respect to a stoichiometric air-fuel ratio so as to set the oxygen storing amount of the catalyst to be “0”. Thereafter, the air-fuel ratio of the gas is controlled to be a predetermined lean air-fuel ratio that is on the leaner side with respect to the stoichiometric air-fuel ratio. The amount of oxygen flowing into the catalyst per a unit time is totaled over a period from the time when the oxygen storing quantity of the catalyst reaches the maximum oxygen storing quantity to the time when the output from an air-fuel-ratio sensor at the downstream side of the catalyst changes to a value showing the lean state, whereby the maximum oxygen storing quantity is estimated. Alternatively, the air-fuel ratio of the gas flowing into the catalyst is controlled to be the aforesaid predetermined lean air-fuel ratio so as to set the oxygen storing quantity to the maximum oxygen storing quantity, and then, the air-fuel ratio of the gas is controlled to be the aforesaid rich air-fuel ratio. The amount of oxygen emitted (consumed) per a unit time in the catalyst is totaled over a period from the time when the oxygen storing quantity of the catalyst becomes “0” to the time when the output from the air-fuel-ratio sensor at the downstream side of the catalyst is changed to the value showing the rich state, whereby the maximum oxygen storing quantity is estimated. The degree of deterioration of the catalyst is acquired on the basis of the maximum oxygen storing quantity thus estimated. The state in which “the output from the air-fuel-ratio sensor indicates the lean state (or rich state)” means the state indicating that the output from the air-fuel-ratio sensor is the air-fuel ratio on the leaner (or richer) side with respect to the stoichiometric air-fuel ratio. The control for forcibly changing the air-fuel ratio of the gas flowing into the catalyst to either one of the predetermined rich air-fuel ratio and the predetermined lean air-fuel ratio from the other as described is referred to as “air-fuel-ratio active control” below.
Meanwhile, the degree of the response to the change (changeover) of the output from the air-fuel-ratio sensor with respect to the change in the air-fuel ratio of the exhaust gas from either one of the air-fuel ratio on the rich side with respect to the stoichiometric air-fuel ratio and the air-fuel ratio on the lean side with respect to the stoichiometric air-fuel ratio to the other has a characteristic of being likely to vary depending upon the property of the air-fuel-ratio sensor itself, deterioration, or the like. Therefore, the point when the output from the air-fuel-ratio sensor at the downstream side of the catalyst is changed to the value showing the lean state or to the value showing the rich state is likely to be deviated from the time when the oxygen storing quantity of the catalyst reaches the maximum oxygen storing quantity or becomes “0”. Accordingly, an error is produced in the estimation of the maximum oxygen storing quantity, which entails a problem that it is difficult to correctly acquire the degree of deterioration of the catalyst.